Faraday rotator and optical attenuator

ABSTRACT

A Faraday rotator and an optical attenuator using the Faraday rotator in which both a fixed magnetic field parallel to and a valuable magnetic field perpendicular to the optical axis are applied to Faraday elements, said optical axis being in the &lt;111&gt; direction of single crystal of garnet, characterized in that three single crystals of garnet of substantially the same thickness having the Faraday effect are used to form Faraday elements and the Faraday elements are arranged in such a manner that a variable magnetic field is applied to one of the Faraday elements, over a range extending 5 deg. each to the left and right of the line connecting the (111) plane in the center of the stereographic projection chart with the (−1−12) plane on the outermost circumference or a plane equivalent thereto in the chart, whereas a variable magnetic field is applied to the remaining two elements, over a range extending 5 deg. each to the left and right of the line connecting the (111) plane in the center of the stereographic projection chart with the (−101) plane on the outermost circumference or a plane equivalent thereto in the chart. Temperature dependence of optical decay thus is improved. Also, positioning means for Faraday rotator improves the polarization dependence loss.

INDUSTRIAL FIELD OF THE INVENTION

[0001] This invention relates to a device for adjusting the angle ofFaraday rotation (Faraday rotator) and also to an optical attenuatorusing such a device. The Faraday rotation angle-adjusting device andoptical attenuator according to the present invention are specificallyused in optical transmission communication systems. The invention, inparticular, improves the temperature dependence of Faraday rotationangle and narrows the scatter of variations among products in the angleof Faraday rotation with externally applied variable magnetic fields.Moreover, the invention reduces the variable magnetic field required toachieve a desired low-current characteristic or specific amount ofattenuation (proportional to the angle of Faraday rotation).

PRIOR ART

[0002] Owing to the striking expansion of transmission capacities, thereis a growing demand for high-density-wavelength multiplex transmissionsystems. This has accordingly increased the need for variable opticalattenuators that dynamically adjust the quantity of light required forthe systems. Applications for the attenuators include the control oflight quantities for individual channels and simultaneous attenuation ofmultiplex light rays. Among those optical attenuators there is one typethat utilizes magneto-optical effect. That type usually involves alayout in which a component capable of changing the angle of Faradayrotation a light beam is disposed between a polarizer and an analyzer.With one such component for changing the Faraday rotation angle,external magnetic fields are applied from two or more differentdirections to a single crystal of garnet having a Faraday effect so asto make the composite external field variable, whereby the Faradayrotation angle of the light that passes through the single crystal ofgarnet is controlled (Registered Japanese Patent 2,815,509).

[0003] To be more specific, the Registered Japanese Patent 2,815,509discloses an optical attenuator which, while keeping a fixed magneticfield greater than the saturation magnetic field of a single crystal ofgarnet applied to the crystal in a direction parallel to the opticalaxis by means of a permanent magnet, applies a variable magnetic fieldto the crystal in a direction perpendicular to the optical axis by anelectromagnet, thereby changes the composite magnetic field vector, andchanges the direction of magnetization and hence the Faraday rotationangle of the single garnet crystal so that the quantity of light coupledto the fiber on the leaving side can be controlled.

[0004] Another method is known as a means of decreasing the temperaturedependence of optical attenuators, which comprises applying externalmagnetic fields in the directions where the amount of change of theFaraday rotation angle due to the temperature dependence of the anglebetween the direction of magnetization of Faraday elements and thedirection of the beam and the amount of change of the Faraday rotationangle due to the temperature dependence with the Faraday effect aredifferent in code from each other and the absolute value of eitheramount is less than twice that of the other amount, whereby the changesof Faraday rotation angle with temperature are restricted. (JapanesePatent Application Kokai No. 11-249095)

PROBLEMS THAT THE INVENTION IS TO SOLVE

[0005] With the foregoing in view, a plurality of optical attenuatorswere experimentally fabricated. The experiments presented a problem ofwide scatter among the specimens of the temperature dependence values ofthe attenuation and the electromagnet field perpendicular to the opticalaxis required to attain the maximum attenuation. Another problem thatarose was the requirement of a large variable magnetic field (and hencea large driving current) to achieve a desired attenuation (proportionalto the angle of Faraday rotation).

[0006] Therefore, the solution of these problems in accordance with thepresent invention is to provide an optical attenuator that narrows thescatter of characteristics among devices, reduces the temperaturedependence, and possesses good low-current characteristics.

[0007] Also, in view of the foregoing, we experimentally fabricated aplurality of optical attenuators and found that they show, in common,that an increase in the quantity of attenuation is accompanied with anincrease in the polarization dependence loss (hereinafter abbreviated to“PDL”) up to more than one decibel at peak. The value is by far thegreater than those of non-polarization-dependent optical isolators, theoptical devices that similarly take the advantage of magneto-opticaleffect and are already in wide use with optical transmission systems.Typical PDL values of the non-polarization-dependent optical isolatorsare of the order of 0.1 dB.

[0008] A study of the difference revealed that, when birefringentelements are used as a polarizer and an analyzer, ordinary andextraordinary rays pass through a Faraday element along different pathsand accordingly the distributions of magnetic fields that are applied todifferent portions of the Faraday element vary too. Factors that cansubtly influence the magnetic field distributions are, for example, thedirection of application of variable magnetic field, shape and size ofthe yoke of an electromagnet that produces the variable field, and therelative positions of the yoke and Faraday element.

[0009]FIG. 22 illustrates, by way of example, a conventional member forattaching a Faraday element to an optical attenuator. The member has acutout 20 for receiving a yoke and an opening 15 formed in the centerfor the passage of a light ray. A Faraday element recess 21 is formed atthe bottom of the cutout 20 in alignment with the opening 15, and aFaraday element (not shown) consisting of one or more garnet crystalplates is received in the recess. The Faraday element is secured inplace with a thermosetting or ultraviolet-curing resin filled inresin-filling ports 16, 17 open to the cutout 20. Yokes 10, 10 ofelectromagnets are inserted on both sides of the opening 15 into thecutout 20 and are similarly fixed with a hardening resin. There is nomeans of positioning the inner ends of the yokes 10 of electromagnets,and the yokes simply secured with respect to the Faraday element fail tomaintain a constant positional relationship. Moreover, the combined areaof the yokes relative to the Faraday element is restricted. These andother factors are deemed responsible for the problem of increased PDL.

[0010] Therefore, another problem that the present invention is to solveare settled by the provision of an optical attenuator having favorablePDL characteristics.

MEANS OF SOLVING THE PROBLEMS

[0011] The present invention provides a Faraday rotator in whichmagnetic field is applied to Faraday elements, optical axis of saidFaraday elements being in the <111> direction of single crystal ofgarnet, characterized in that three single crystals of garnet ofsubstantially the same thickness having a Faraday effect are used toform the Faraday elements and the Faraday elements are arranged in sucha manner that a first magnetic field is applied to one of the Faradayelements, in the direction perpendicular to a plane in a range extending5 deg. each to the left and right of the line connecting the (111) planein the center of a stereographic projection chart with the (−1−12) planeon the outermost circumference or a plane equivalent thereto in thechart, whereas a second magnetic field is applied to the remaining twoelements, in the direction perpendicular to plane in a range extending 5deg. each to the left and right of the line connecting the (111) planein the center of the stereographic projection chart with the (−101)plane on the outermost circumference or a plane equivalent thereto inthe chart. Each of said first and second magnetic fields may be acomposite magnetic field formed by a pair of magnetic fields.

[0012] The present invention also provides a Faraday rotator in whichboth a magnetic field parallel to and a magnetic field perpendicular tothe optical axis are applied to Faraday elements, said optical axisbeing in the <111> direction of single crystal of garnet, characterizedin that

[0013] three single crystals of garnet of substantially the samethickness having a Faraday effect are used to form the Faraday elementsand the Faraday elements are arranged in such a manner that a firstmagnetic field is applied to one of the Faraday elements, in thedirection perpendicular to a plane in a range extending 5 deg. each tothe left and right of the line connecting the (111) plane in the centerof the stereographic projection chart with the (−1−12) plane on theoutermost circumference or a plane equivalent thereto in the chart,whereas a second magnetic field is applied to the remaining twoelements, in the direction perpendicular to a plane in a range extending5 deg. each to the left and right of the line connecting the (111) planein the center of the stereographic projection chart with the (−101)plane on the outermost circumference or a plane equivalent thereto inthe chart.

[0014] The present invention also provides an optical attenuatorincluding a polarizer and an analyzer disposed, respectively, before andafter a plurality of Faraday elements, in which a variable magneticfield is applied to Faraday elements, in such manner that the variablemagnetic field can change the angle of Faraday rotation of a light beamand control the quantity of light transmitted, said optical axis beingin the <111> direction of single crystal of garnet, characterized inthat

[0015] three single crystals of garnet of substantially the samethickness having a Faraday effect are used to form Faraday elements andthe Faraday elements are arranged in such a manner that a variablemagnetic field is applied to one of the Faraday elements, in thedirection perpendicular to planes over a range extending 5 deg. each tothe left and right of the line connecting the (111) plane in the centerof the stereographic projection chart with the (−1−12) plane on theoutermost circumference or a plane equivalent thereto in the chart,whereas a variable magnetic field is applied to the remaining twoelements, in the direction perpendicular to planes over a rangeextending 5 deg. each to the left and right of the line connecting the(111) plane in the center of the stereographic projection chart with the(−101) plane on the outermost circumference or a plane equivalentthereto in the chart.

[0016] The variable magnetic field may be a composite magnetic fieldapplied from a pair of magnetic fields at least one of which isvariable.

[0017] The present invention also provides an optical attenuatorincluding a polarizer and an analyzer disposed, respectively, before andafter a plurality of Faraday elements, in which both a magnetic fieldparallel to and a magnetic field perpendicular to the optical axis areapplied to the Faraday elements, one of the magnetic fields being fixedand the other being variable, so that the composite magnetic fieldthereof can change the angle of Faraday rotation of a light beam andcontrol the quantity of light transmitted, said optical axis being inthe <111> direction of single crystal of garnet, characterized in that

[0018] three single crystals of garnet of substantially the samethickness having a Faraday effect are used to form Faraday elements andthe Faraday elements are arranged in such a manner that a variablemagnetic field is applied to one of the Faraday elements, in thedirection perpendicular to planes over a range extending deg. each tothe left and right of the line connecting the (111) plane in the centerof the stereographic projection chart with the (−1−12) plane on theoutermost circumference or a plane equivalent thereto in the chart,whereas a variable magnetic field is applied to the remaining twoelements, in the direction perpendicular to planes over a rangeextending 5 deg. each to the left and right of the line connecting the(111) plane in the center of the stereographic projection chart with the(−101) plane on the outermost circumference or a plane equivalentthereto in the chart.

[0019] In the Faraday rotator and optical attenuator defined above,preferably the magnetic field parallel to the optical axis is a fixedmagnetic field generated by permanent magnets and the magnetic fieldperpendicular to the optical axis is a variable magnetic field generatedby electromagnets.

[0020] According to the invention, the Faraday rotator is composed of acombination of three Faraday elements of specific orientation, and notonly the temperature dependence of Faraday rotation angle is reduced butalso the scatter of Faraday rotation characteristics is controlled.

[0021] To be more specific, the Faraday rotator and attenuator accordingto the present invention are limited in temperature variation of theattenuation (that depends on the Faraday rotation angle) and, moreover,the scatter of the attenuation-temperature variation among the specimensis narrow. Furthermore, the scatter of the peak current value(corresponding to the variable magnetic field required to achieve themaximum change in the Faraday rotation angle) among the specimens issmall and, in addition, low-current characteristics can be attained(that is, the driving current to generate a variable magnetic fieldnecessary for obtaining a certain amount of attenuation or Faradayrotation angle may be small).

ANOTHER MEANS OF SOLVING THE PROBLEMS

[0022] The present invention also provides an optical attenuator whichcontrols the angle of Faraday rotation of a light beam that passesthrough a single crystal of garnet having the Faraday effect by applyingtwo external magnetic fields, fixed and variable, from two differentdirections, characterized in that at least one garnet crystal having theFaraday effect is used as a Faraday element, and a member for holdingthe Faraday element in place has a stopper to position the front ends ofyokes of electromagnets that apply the variable magnetic field to andaround the holder, with respect to the direction of field application.

[0023] The invention also provides an optical attenuator as definedabove characterized in that the member for holding the Faraday elementin place has a pair of positioning grooves to position the front ends ofyokes of electromagnets that apply the variable magnetic field to andaround the holder, with respect to the direction of light beam.

[0024] The invention further provides an optical attenuator according toclaim 1 characterized in that the yokes of the electromagnets that applythe variable magnetic field have a front end plane each perpendicular tothe direction of the variable field with a cross sectional area no lessthan 1.7 times that of the plane of the Faraday element perpendicular tothe direction of the variable field.

[0025] To be more concrete, the invention provides an optical attenuatorcomprising a member formed with a first groove extending across theoptical axis and also formed with an opening open to the first groovealong the optical axis, a Faraday element disposed in the first groovein alignment with the optical axis, said member having a pair of secondgrooves formed on both sides of, and close to, the Faraday element, saidsecond grooves extending across the first groove and in the directionnormal to the optical axis, and a pair of electromagnets that produces avariable magnetic field, said magnets having yokes the ends of which arefitted in the pair of second grooves on both sides of the Faradayelement, the bottoms of the second grooves serving as a stopper forpositioning the front ends of the yokes.

BRIEF EXPLANATION OF THE DRAWINGS

[0026]FIG. 1 is a stereographic projection chart of crystal facescentered on the (111) plane.

[0027]FIG. 2 is a graphic representation of the relation between theelectromagnet field and Faraday rotation angle in the route ofmagnetization rotation.

[0028]FIG. 3 is a perspective view showing the construction of anoptical attenuator.

[0029]FIG. 4 is a schematic view of a 1-mm cubic single crystal ofgarnet as beveled.

[0030]FIG. 5 gives graphs showing the characteristics of opticalattenuator specimens, i.e., their values of temperature dependence onattenuation-current characteristics values, when a variable magneticfield was applied to one of the three single crystals of garnet in eachspecimen, over the line connecting the (111) plane in the center withthe (−1−12) plane on the outermost circumference of the stereographicprojection chart, whereas a variable magnetic field was applied to theremaining two elements, over the line connecting the (111) plane in thecenter with the (−101) plane on the outermost circumference of thechart.

[0031]FIG. 6 is a table giving the characteristics of optical attenuatorspecimens, i.e., their peak current values and maximum values ofattenuation variation over the temperature range of 0-65° C., when avariable magnetic field was applied to one of the three single crystalsof garnet in each specimen, over the line connecting the (111) plane inthe center with the (−1−12) plane on the outermost circumference of thestereographic projection chart, whereas a variable magnetic field wasapplied to the remaining two elements, over the line connecting the(111) plane in the center with the (−101) plane on the outermostcircumference of the chart.

[0032]FIG. 7 gives graphs showing the characteristics of opticalattenuator specimens, i.e., their values of temperature dependence onattenuation-current characteristics values, when a variable magneticfield was applied to all three Faraday elements of each specimen, on theline connecting the (111) plane in the center with a plane inclined atan angle of 26° from the (−1−12) plane on the outermost circumferencetoward the (−101) plane of the stereographic projection chart.

[0033]FIG. 8 is a table giving the characteristics of optical attenuatorspecimens, i.e., their peak current values and maximum values ofattenuation variation over the temperature range of 0-65° C., when avariable magnetic field was applied to all three Faraday elements ofeach specimen, on the line connecting the (111) plane in the center witha plane inclined at an angle of 26° from the (−1−12) plane on theoutermost circumference toward the (−101) plane of the stereographicprojection chart.

[0034]FIG. 9 gives graphs showing the characteristics of opticalattenuator specimens, i.e., their values of temperature dependence onattenuation-current characteristics values, when a variable magneticfield was applied to one of the three single crystals of garnet in eachspecimen, over the line connecting the (111) plane in the center withthe (−1−12) plane on the outermost circumference of the stereographicprojection chart, whereas a variable magnetic field was applied to theremaining two elements, over the line connecting the (111) plane in thecenter with the (−211) plane on the outermost circumference of thechart.

[0035]FIG. 10 is a table giving the characteristics of opticalattenuator specimens, i.e., their peak current values and maximum valuesof attenuation variation over the temperature range of 0-65° C., when avariable magnetic field was applied to one of the three single crystalsof garnet in each specimen, over the line connecting the (111) plane inthe center with the (−1−12) plane on the outermost circumference of thestereographic projection chart, whereas a variable magnetic field wasapplied to the remaining two elements, over the line connecting the(111) plane in the center with the (−211) plane on the outermostcircumference of the chart.

[0036]FIG. 11 shows the temperature dependence characteristics of atotal of 444 optical attenuators fabricated from 16 lots of garnetcrystals in accordance with the procedures described in the examples ofthe invention.

[0037]FIG. 12 is a schematic view showing the construction of an opticalattenuator.

[0038]FIG. 13 gives the results of computation of the relation betweenPDL and the discrepancy between Faraday rotation angles of split lightbeams and PDL.

[0039]FIG. 14 shows the field application zone of electromagnets thatapply a variable magnetic field.

[0040]FIG. 15 gives the results of computation of magnetic fielddistribution in a field application zone.

[0041]FIG. 16 illustrates samples of a symmetrical pair of electromagnetyokes and an asymmetrical pair of electromagnet yokes.

[0042]FIG. 17 gives the results of evaluation of the characteristics ofdistribution within the incidence area of PDL and attenuation insymmetrical and asymmetrical electromagnet yoke samples.

[0043]FIG. 18 depicts an element holder having a yoke stopper andyoke-positioning grooves; (a) being a left side view, (b) a front view,(c) a back view, (d) a plan view, and (e) a bottom view.

[0044]FIG. 19 presents schematic views showing how an element is joinedto an element holder and how a pair of yokes are joined under pressure;(a) being a left side view, (b) a back view, and (c) a perspective viewas seen from below the rear side.

[0045]FIG. 20 is a view illustrating single crystal of garnet onemillimeter square in size after a corner beveling.

[0046]FIG. 21 gives characteristics, i.e., PDL and attenuation currentcharacteristics and PDL values at the attenuation of 18.5 dB, of samplesembodying the present invention.

[0047]FIG. 22 illustrates an element holder not provided with a yokestopper; (a) being a right side view, (b) a back view, (c) a plan view,and (d) a bottom view.

[0048]FIG. 23 gives characteristics, i.e., PDL and attenuation currentcharacteristics and PDL values at the attenuation of 18.5 dB, of samplesin comparative examples.

PREFERRED EMBODIMENTS OF THE INVENTION

[0049] In the following, preferred embodiments are described. It isnoted that the embodiments are described in connection with a compositemagnetic field composed of two magnetic fields, one being fixed and theother being variable, it should be noted that so long as the desiredvector of variable magnetic field may be generated a singleelectromagnet or a plurality of electromagnets, or a combination ofpermanents and electromagnets may be adopted. In the following, itshould be understood that although the direction of the compositemagnetic field is not specified the direction is in perpendicular to theplane designated by the crystallographic planes. That is, a statementthat a variable magnetic field is applied in over a line connecting the(111) plane to (k,l,m) (k, l, m are specific integers) means that themagnetic field is applied in the direction between <111> (inclusive) and<k,l,m> (inclusive).

[0050] In accordance with the present invention, Faraday elements arearranged in such a way that a variable magnetic field of electromagnetsis applied to one of three elements, over the line connecting the (111)plane in the center with the (−1−12) plane on the outermostcircumference of the stereographic projection chart, whereas a variablemagnetic field is applied to the remaining two elements, over the lineconnecting the (111) plane in the center with the (−101) plane on theoutermost circumference of the chart. FIG. 1 is a stereographicprojection chart centered on the (111) plane of a single crystal ofgarnet. Any given plane of the garnet crystal may be represented by adot in this stereographic projection chart. Owing to the symmetry of thecrystal structure, a plane equivalent to the (−1−12) plane emerges atevery 120 deg. on the outermost circumference, and a plane equivalent tothe (−101) plane comes at every 60 deg. Here the plane equivalent to the(−1−12) plane is either the (−12−1) or (2−1−1) plane and the planeequivalent to the (−101) plane is any of the (−110), (01−1), (10−1),(1−10), and (0−11) planes. Negative indices of the crystal planes areindicated by indices each with a minus symbol.

[0051]FIG. 2 graphically represents the results of measurement ofFaraday rotation angle and magnetic field in the directions ofelectromagnet field application with different garnet crystalorientations. The graph reveals that the Faraday rotation angle varieswidely with the electromagnet field depending on the direction ofmagnetic field application. Thus the reproducibility of attenuationcharacteristics with the applied variable magnetic field can be enhancedby distinctly specifying the relation between the orientation of garnetsingle crystal and the direction of electromagnet field applicable tothe garnet crystal. It has now been found possible to reduce with goodreproducibility the temperature dependence of the attenuationcharacteristics with the applied variable magnetic field, when, inconformity with the invention, a variable magnetic field is applied toone of the three single crystals of garnet in each specimen, over theline connecting the (111) plane in the center with the (−1−12) plane onthe outermost circumference of the stereographic projection chart,whereas a variable magnetic field is applied to the remaining twoelements, over the line connecting the (111) plane in the center withthe (−101) plane on the outermost circumference of the chart.

[0052] As will be clear from a comparison between the Examples of theinvention and Comparative Examples to be given below, the narrowing ofscatter of the temperature dependence of attenuation according to theinvention is presumably attributable to the greater tolerance than inthe prior art of the angular deviation of different directions ofmagnetic fields that are applied to specific orientations.

[0053]FIG. 3 illustrates the basic construction of a Faraday rotatorcomprising a combination of three Faraday elements and two differentmagnets and of an optical attenuator using the rotator. The arrangementis such that a light beam travels, from the beam incidence side forward,through a polarizer 2, a set of three Faraday elements 3, 4, 5 asdefined above, an analyzer 6, and a phase compensation prism 8, in theorder of mention. To the Faraday elements 3, 4, 5 is applied a fixedsaturation magnetic field in the optical axis by permanent magnets 7, 7.Also, a variable magnetic field is applicable in the direction normal tothe optical axis by electromagnets 10,10. These three Faraday elementsand two kinds of magnets are joined to constitute a Faraday rotator.

EXAMPLE 1

[0054] An optical attenuator of the basic construction shown in FIG. 3was made, and the relation between the direction of application ofelectromagnet field and the orientation of garnet crystal, the relationbetween the amount of attenuation and electromagnet field, and thetemperature dependency were examined. The arrangement was such that abeam of light could pass through a polarizer, a plurality of Faradayelements, an analyzer, and a phase compensation prism, located in theorder of mention. The Faraday elements were arranged so that the lightbeam was incident perpendicularly to the (111) planes of the elements. Amagnetic field parallel to the light beam was applied by two permanentmagnets to the Faraday elements, while electromagnets applied a magneticfield perpendicular to the beam. As the Faraday rotator was kept in thestate of magnetic saturation, the current being supplied to theelectromagnets was varied so as to change continuously the angle ofFaraday rotation of the beam being transmitted and accordingly changethe quantity of light of the leaving beam. The relative angle of thepolarization planes of the polarizer and analyzer through which the beamwas to pass was set to 105 deg.

[0055] The Faraday element was fabricated in the following way. A singlecrystal of garnet was grown on a nonmagnetic garnet substrate by theliquid phase epitaxial technique. With reference to the orientation flatsurface formed on the nonmagnetic garnet substrate, the resultingcrystal was slitted at intervals of 11 mm in parallel with andperpendicularly to the <−1−12> direction, and the upper right corner ofthe side of each slitted piece normal to the <−1−12> direction wasbeveled. Next, the substrate was removed by grinding, and thesemifinished pieces were heat treated at 1030° C. in air for 20 hours.The heat treatment was done for the purpose of reducing the growthinduced magnetic anisotropy. The pieces were then mirror polished to afinish thickness at which the angle of Faraday rotation was about 32deg. Following this, nonreflective films were formed on both sides ofthe pieces. Next, the 11 mm-square garnet single crystal pieces formedwith the nonreflective films were cut into chips 1 mm square in thedirections parallel to and perpendicular to the four sides of eachpiece. The upper right of the side perpendicular to the <−1−12>direction of each chip was beveled (FIG. 4). The beveling was intendedto make the crystal orientation of each chip after the scissiondistinct. Three such 1 mm-square chips of garnet single crystal wereused as Faraday elements.

[0056] Two optical attenuators each of three different garnet singlecrystal lots were made. In each attenuator Faraday elements werearranged in such a way that a variable magnetic-field was applicable toone of the three elements, over the line connecting the (111) plane inthe center with the (−1−12) plane on the outermost circumference of thestereographic projection chart, whereas a variable magnetic field wasapplicable to the remaining two elements, over the line connecting the(111) plane in the center with the (−101) plane on the outermostcircumference of the chart.

[0057] The temperature dependence values (at 0°, 25°, and 65° C.) of theattenuation-current characteristics of these test specimens weremeasured. The results are shown in FIG. 5. The attenuation-currentcharacteristics and temperature dependence tendencies were quitefavorably reproduced by the individual specimens. The graphs alsoindicate the attenuation-temperature variations as computed from themeasured values of attenuation.

[0058] With each specimen the peak current at which the peak ofattenuation at 25° C. was attained and the maximum value of thevariation in attenuation over the range of 0-65° C. at attenuation below20 dB were determined. The results are summarized in FIG. 6. With theindividual samples the attenuation-current characteristics andtemperature dependence values were reproduced with very good results.

COMPARATIVE EXAMPLE 1

[0059] As Comparative Example 1, optical attenuators were made with aFaraday element arrangement such that electromagnet fields wereapplicable in different orientations. The Faraday elements were arrangedso that all three were superposed in the same orientation and anelectromagnet field was applicable to the line connecting the (111)plane in the center with a plane inclined at an angle of 26° from the(−1−12) plane on the outermost circumference toward the (−101) plane ofthe stereographic projection chart. Three optical attenuators were madeeach from two different garnet single crystal lots, and the temperaturedependence values (at 0°, 25°, and 65° C.) of the attenuation-currentcharacteristics of these test specimens were measured. The results areshown in FIG. 7.

[0060] With each specimen the peak current at which the peak ofattenuation at 25° C. was attained and the maximum value of thevariation in attenuation over the range of 0-65° C. at attenuation below20 dB were determined. The results are summarized in FIG. 8. The graphsindicate that, with the individual samples, the peak current andtemperature dependence varies widely from specimen to specimen.

[0061] It is obvious from the foregoing that in the Example of thepresent invention not only the attenuation-temperature variation islimited but also the scatter of the attenuation-temperature variationamong the specimens tested was small.

[0062] In addition, the scatter of peak current values (corresponding tothe variable magnetic fields required to obtain the maximum change inthe angle of Faraday rotation) is restricted and, moreover, low-currentcharacteristics are obtained (which means that the amount of a drivingcurrent to produce a variable magnetic field required to obtain aspecific attenuation amount or angle of Faraday rotation can be keptsmall).

COMPARATIVE EXAMPLE 2

[0063] As Comparative Example 2, a plurality of optical attenuators ofvaried garnet single crystal lots were made. In each attenuator Faradayelements were arranged in such a way that a variable magnetic field wasapplicable to one of the three elements, over the line connecting the(111) plane in the center with the (−1−12) plane on the outermostcircumference of the stereographic projection chart, whereas a variablemagnetic field was applicable to the remaining two elements, over theline connecting the (111) plane in the center with the (−211) plane onthe outermost circumference of the chart. The temperature dependencevalues (at 0°, 25°, and 65° C.) of the attenuation-currentcharacteristics of these test specimens were measured. The results areshown in FIG. 9. With each specimen the peak current at which the peakof attenuation at 25° C. was attained and the maximum value of thevariation in attenuation over the range of 0-65° C. at attenuation below20 dB were determined. The results are summarized in FIG. 10. Althoughthe individual samples gave favorable values of temperature dependenceupon attenuation, the peak-current value at which the maximumattenuation was achieved was about 70 mA, as much as about 1.8 timesgreater than the values of the specimens fabricated in accordance withthe present invention, suggesting the effectiveness of the presentinvention in lowering the current requirement.

EXAMPLE 2

[0064] Following the procedure described in Example 1 of the presentinvention, a total of 444 optical attenuators were made from 16 lots ofgarnet crystals. Their temperature dependence values were determined,the results being summarized in FIG. 11. Despite the possibility ofscatter of approximately ±5 deg. in crystal orientation as the scatterof fabrication including the tolerances in the cutting direction and insecuring the elements in place, the figure indicates narrow scatter oftemperature dependence and favorable reproducibility.

[0065] As has been described above in connection with FIGS. 1-11, thepresent invention improves the reproducibility of attenuationcharacteristics of optical attenuators with respect to applied variablemagnetic fields, reduces the temperature dependence of the attenuationcharacteristics with respect to applied variable fields, and enhancesthe reproducibility. Further, the invention makes it possible to achievelow-current characteristics.

[0066] The present invention is also concerned with an opticalattenuator which controls the quantity of light that transmits through asingle crystal of garnet having a Faraday effect by applying twoexternal magnetic fields, one fixed and the other variable, fromopposite directions to the crystal and thereby making the Faradayrotation angle of the ray of light that transmits through the crystalvariable. By way of example, a basic construction of an opticalattenuator is shown in FIG. 12. The arrangement is such that a beam oflight passes through a polarizer 2, a plurality of Faraday elements 1each consisting of a garnet crystal, an analyzer 6, and a phasecompensation prism 8, located in the order of mention, so that anattenuated beam of light emerges as indicated by an arrow. Externalmagnetic field application means comprises a pair of permanent magnets7, 7 disposed on opposite sides of the Faraday elements 1 and whichjointly apply a magnetic field parallel to the axis of light and a pairof electromagnets 10, 10 (only the front ends of their yokes beingshown) which apply a variable magnetic field perpendicular to the lightaxis. In order to attain independence from polarization, wedge-shapedpolarization separation elements are used for the polarizer 2 andanalyzer 6. Those elements are made of birefringent crystals. As aconsequence, the incoming light beam is separated into ordinary andextraordinary rays in the polarizer 2 located on the incidence side ofthe Faraday elements (garnet crystals), and then in the polarized statethe separate rays enter the garnet crystals. As the rays travel throughthe garnet crystals, their directions of polarization are rotated by theFaraday effect. This behavior is taught in the Registered U.S. Pat. No.2,815,509. In brief, as they travel through the garnet crystals, theordinary and extraordinary rays separated as a result of polarizationpass different paths across the crystals. Thus, theoretically, differentFaraday rotation angles of the two rays separated by polarization causea PDL.

[0067] In FIG. 13 are graphically shown the results of calculation ofthe relation between PDL and the discrepancy between separated rays withan attenuation value of 18.5 dB. The relation is represented by amathematical expression or Formula 1;

[0068] where φ is the relative angle of the optical axis of thewedge-shaped polarization separation elements, Δθ is the discrepancybetween the Faraday rotation angles of ordinary and extraordinary rays,Att is the attenuation value, and θfAtt is the Faraday rotation angle atwhich a desired attenuation value is attained:

PDL=|−10 log └cos²{φ−(θ_(fAtt)+Δθ)}┘−Att|  [Formula 1]

[0069] The formula indicates that the PDL increases as the discrepancybetween the Faraday rotation angles expands.

[0070] Ordinarily the angle of Faraday rotation varies with the externalmagnetic field that is applied to the single crystals of garnet. Toreduce the discrepancy between the angles of Faraday rotation of therays separated by polarization, therefore, it is necessary to apply asuniform a magnetic field to the crystals as possible. In reality,however, the strength of the variable magnetic field that is applied tothe electromagnets is difficult to control, because the strength dependson various factors such as the size and shape of the yokes of theelectromagnets and their relative position to the garnet crystals.

[0071] A magnetic field application zone of electromagnets that apply avariable magnetic field (i.e., the zone where Faraday elements aredisposed) is shown in FIG. 14. A variable current is passed throughcoils 11 of conductor wire wound on both yokes 10 to produce a variablemagnetic field between the yokes. The variable magnetic field is appliedto one or more Faraday elements of garnet crystal interposed in theregion between the yokes so as to adjust the attenuation value of light.

[0072] The angular distribution of the vector of the external magneticfield applied to this region was computed. The results are shown in FIG.15. As conditions for the computation, the distance between the yokes10, 10 of the two electromagnets was set to 1 mm, the cross sectionalarea of the front end of each yoke to 1×1 mm, the number of turns ofwire in each coil to 1,500 turns, the diameter of the wire to 0.1 mm,and the electromagnet current to 70 mA. A model arrangement was made inwhich two permanent magnets 7, 7 were located at the front and rear ofthe yokes to apply a magnetic field parallel to the optical axis (seeFIG. 12). The dimensions of the permanent magnets were 3.5 mm in outsidediameter, 1.3 mm in inside diameter, and 1.0 mm long. The distancebetween the center of each yoke and the center of each permanent magnetwas 3.5 mm. It will be appreciated that the most favorably distributedregion is at and around the centers. For this reason it is presumedthat, for the reduction of the PDL characteristic, the element should belocated as precisely in the center between the electromagnet yokes aspossible.

[0073] The above was experimentally confirmed. Experiments were made tosee if the PDL characteristic is improved in a uniform magnetic field.Two sample models of optical attenuators, one having symmetricalelectromagnetic yokes and the other asymmetrical electromagnetic yokes,were made. Each sample was evaluated in respect of the distributions ofPDL and attenuation in the incidental plane. The symmetricalelectromagnet yoke sample used left and right yokes arrangedsymmetrically and the asymmetrical electromagnet yoke sample used oneyoke located about 100 μm more distant from the other yoke.

[0074] The evaluation results are graphically represented in FIG. 17. Tosum up, the closer to the center between the yokes the less theattenuation was. With the asymmetrical yoke sample, the minimum peak wasabout 100 μm off the center. The results were in agreement with theresults of magnetic field analysis in which the closer to the centerbetween the yokes the smaller the absolute value of the magnetic field.The PDL distribution too became less as it approaches the center betweenthe electromagnets. As with the attenuation, the minimum peak was alsoabout 100 μm off the center. This indicates that in the center betweenelectromagnets where the magnetic field distribution is favorable, PDLtoo is improved.

[0075] From the foregoing it is clear that a favorable distribution ofthe externally applied magnetic field in the Faraday elements isessential for the improvement in the PDL characteristic. For theimproved field distribution it is also effective to locate the elementas precisely in the center between the electromagnet yokes as possibleand allow the front ends of the magnet yokes to have a cross sectionalarea greater than that of the Faraday element.

[0076] In addition, the provision of a member for holding the Faradayelement in position and of a stopper formed in the periphery of themember for the insertion of yokes improves the symmetry of theelectromagnet yokes and enhance the PDL characteristic value.

[0077] Further, the provision of the element-holding member formed withgrooves in the periphery of the member to position the electromagnetyokes with respect to the optical axis makes it possible for the Faradayelement to be disposed in the center between the electromagnet withrespect to the optical axis direction. With the yokes of theelectromagnets that apply the variable magnetic field, the larger thecross sectional area of the plane of each yoke end perpendicular to thedirection of the variable field compared with the cross sectional areaof the Faraday element perpendicular to the variable field, the betterthe uniformity of the magnetic field to which the Faraday element issubjected.

[0078] Moreover, the stopper formed in the element holder keeps theyokes out of contact with the element, enhancing the reliability againstthermal expansion due to temperature changes and against changes withthe lapse of time.

[0079] An optical attenuator of the basic construction illustrated inFIG. 12 was fabricated. The construction was such that a light beamindicated by an arrow could pass through a polarizer 2, a Faradayelement 1 consisting of three plates of garnet crystal, an analyzer 6,and a phase compensation prism 8 to yield an attenuated light beam. TheFaraday element 1 was located in such a way that the light beam could beincident perpendicularly to the (111) plane of the element. To thisFaraday element was applied a magnetic field parallel to the light beamby means of two permanent magnets 7, 7 and a magnetic field normal tothe light beam was applied by electromagnets 10,10. While the Faradayelement 1 was being kept in a magnetically saturated state by means ofthe permanent magnets 7, 7, the current being supplied to theelectromagnets 10, 10 was varied, whereby the angle of Faraday rotationof the transmitted light could be continuously varied and the quantityof the emerging light beam be changed. The relative angle of thepolarization plane of the light passing through the polarizer 2 and theanalyzer 6 each was 105 deg.

[0080] The Faraday element was joined securely to an element-securingholder 12 as shown in FIG. 18. To be more particular, the element holder12 has a first groove 20 which extends across the optical axis, aFaraday element-holding stage 18 formed in the middle portion of thegroove 20, an opening 15 formed in the Faraday element-holding stage 18in the groove 20 and along the optical axis, and a pair ofyoke-positioning grooves 14 formed close to the both sides of theFaraday element-holding stage 18 and extending normal to the opticalaxis across the first groove. With respect to the Faradayelement-holding stage 18, there are two resin injection holes 16, 17formed in the walls of the first groove 20, in alignment with eachother. In the yoke-positioning grooves 14, the portion adjacent to thefirst groove 20 (the portion indicated by two-dot chain lines in FIG.18(a)) constitutes a stopper portion 13 for the yokes. Unlike thecounterpart of the prior art illustrated in FIG. 22, the first groove 20serves as a groove for positioning the Faraday element, where the yokesare held in position by utilizing the yoke-positioning grooves 14 andthe stopper portion 13. Steps formed between the first groove 20 and oneside faces of the yoke-positioning grooves 14 as shown not onlyincreases the area of the yoke stopper portion 13 but also locate theFaraday element in the centers of the yoked in the direction of theoptical axis, thus making it possible to apply a uniform magnetic fieldto the Faraday element.

[0081] As shown in FIG. 19, the Faraday element 1 is fitted in theFaraday element-holding stage 13 in the first groove 19, locat{cuberoot}ing the element in alignment with the optical axis, and a curableresin is injected into the stage through the resin injection holes 16,17 to secure the element in place. Next, the yokes 10 of the twoelectromagnets are fitted in the pair of yoke-positioning grooves 14 soas to sandwich the Faraday element in between. The front ends of theyokes are pressed against the yoke-positioning stopper portion at thebottom of the grooves, and the curable resin is injected into thegrooves 14 to fix the yoke ends securely. In this manner the yoke endsare precisely positioned. Also, the both side walls of theyoke-positioning grooves 14 allow the yokes 10 to be positioned in thedirection of the optical axis.

[0082] Further, as FIG. 18(a) and FIG. 19(b) indicate, the crosssectional area of the stopper portion 13 of the yoke-positioning grooves14 is larger than that of the first groove 20, the distribution of themagnetic field applied to the Faraday element 1 is made all the moreuniform.

[0083] As described above, the element holder according to the presentinvention is utilized in securing electromagnet yokes in place, wherebythe symmetry of electromagnet yokes is enhanced. Moreover, because theyokes are kept out of contact with the element, the qualitativereliability of the assembly is improved. The electromagnet yokes usedfor the experiments had front end dimensions of 1.3 mm by 1.2 mm. Thecross sectional area of the electromagnet yokes was set to a value about1.7 times that of the element.

[0084] The Faraday element was fabricated in the following way. A singlecrystal of garnet was grown on a nonmagnetic garnet substrate by theliquid phase epitaxial technique. With reference to the orientation flatsurface formed on the nonmagnetic garnet substrate, the resultingcrystal was slitted at intervals of 11 mm in parallel with andperpendicularly to the <−1−12> direction, and the upper right corner ofthe side of each slitted piece normal to the <−1−12> direction wasbeveled. Next, the substrate was removed by grinding, and thesemifinished pieces were heat treated at 1030° C. in air for 20 hours.The heat treatment was done for the purpose of reducing the growthinduced magnetic anisotropy. The pieces were then mirror polished to afinish thickness (about 0.3 mm) at which the angle of Faraday rotationis about 32 deg. Following this, nonreflective films were formed on bothsides of the pieces. Next, the 11 mm-square garnet single crystal piecesformed with the nonreflective films were cut into chips 1 mm square inthe directions parallel to and perpendicular to the four sides of eachpiece. The upper right of the side perpendicular to the <−1−12>direction of each chip was beveled (FIG. 20). The beveling is intendedto clarify the crystal orientation of each chip after the scission.Three such 1 mm-square chips of garnet single crystal were used asFaraday elements.

[0085] The three Faraday elements were placed in the element holderhaving yoke stopper, and were bonded securely in position withultraviolet-curing resin injected through the upper and lower holes 0.7mm in diameter each. The Faraday elements were fixed after positioningwith care taken not to allow them to come out of place. In fixing theelectromagnets, they were bonded in place with their yoke ends pressedagainst the yoke stopper of the element holder. This enhanced thesymmetry of the left and right electromagnet yokes and ensured thestability of the yoke-to-yoke distance.

[0086] With the optical attenuators thus experimentally fabricated usingthe element holder provided with a yoke stopper, their PDL andattenuation current characteristics and the PDL values at theattenuation of 18.5 dB were evaluated. The results are given in FIG. 21.As for the PDL current characteristics, the maximum PDL value at thecurrent level that gave the attenuation peak was of the order of 0.5 dB.The PDL value at the attenuation of 18.5 dB, as the average of 14 sampleattenuators fabricated, was 0.25 dB, a favorable characteristic value.

[0087] By way of comparison, optical attenuators were made using anelement holder not provided with a yoke stopper as shown in FIG. 22 andemploying electromagnets with yoke end dimensions of 1.0 mm by 1.2 mm.With these attenuators, the PDL and attenuation current characteristicsand PDL values at an attenuation of 18.5 dB were evaluated. FIG. 23summarizes the results. In respect of the PDL current characteristics,the maximum PDL value at the current level that yielded the attenuationpeak exceeded 1.2 dB. The average PDL value of 9 samples triallymanufactured was 0.53 dB at the attenuation of 18.5 dB. The individualPDL values were about twice the values of the samples made in accordancewith the present invention, and this demonstrates the effectiveness ofthe invention in improving the PDL characteristic.

[0088] As has been described above in connection with FIGS. 12-23, thepresent invention renders the variable magnetic field applicable toFaraday elements uniform and thereby improves the PDL characteristic.[Description of symbols]  1, 3, 4, 5 Faraday elements  2 Polarizer  6Analyzer  7 Permanent magnet  8 Phase compensation prism 10Electromagnet 11 Coil of electromagnet 13 Yoke stopper 14Yoke-positioning groove 15 Opening 16, 17 Resin-filling ports 18Element-holding stage 20 Element holder

1. A Faraday rotator in which a magnetic field is applied to Faradayelements, optical axis of said Faraday elements being in the <111>direction of single crystal of garnet, characterized in that threesingle crystals of garnet of substantially the same thickness having aFaraday effect are used to form the Faraday elements and the Faradayelements are arranged in such a manner that a first magnetic field isapplied to one of the Faraday elements, in the direction perpendicularto a plane in a range extending 5 deg. each to the left and right of theline connecting the (111) plane in the center of a stereographicprojection chart with the (−1−12) plane on the outermost circumferenceor a plane equivalent thereto in the chart, whereas a second magneticfield is applied to the remaining two elements, in the directionperpendicular to a plane in a range extending 5 deg. each to the leftand right of the line connecting the (111) plane in the center of thestereographic projection chart with the (−101) plane on the outermostcircumference or a plane equivalent thereto in the chart.
 2. A Faradayrotator according to claim 1, in which each of said magnetic field is acomposite magnetic field formed by a pair of magnetic fields.
 3. AFaraday rotator in which both a magnetic field parallel to and amagnetic field perpendicular to the optical axis are applied to Faradayelements, one of the magnetic fields being fixed and the other beingvariable, so that a composite magnetic field thereof can change theangle of Faraday rotation of a light beam, said optical axis being inthe <111> direction of single crystal of garnet, characterized in thatthree single crystals of garnet of substantially the same thicknesshaving a Faraday effect are used to form the Faraday elements and theFaraday elements are arranged in such a manner that a variable magneticfield is applied to one of the Faraday elements, in the directionperpendicular to a plane over a range extending 5 deg. each to the leftand right of the line connecting the (111) plane in the center of astereographic projection chart with the (−1−12) plane on the outermostcircumference or a plane equivalent thereto in the chart, whereas avariable magnetic field is applied to the remaining two elements, in thedirection perpendicular to a plane in a range extending 5 deg. each tothe left and right of the line connecting the (111) plane in the centerof the stereographic projection chart with the (−101) plane on theoutermost circumference or a plane equivalent thereto in the chart.
 4. AFaraday rotator according to claim 3 in which the magnetic fieldparallel to the optical axis is a fixed magnetic field generated bypermanent magnets and the magnetic field perpendicular to the opticalaxis is a variable magnetic field generated by electromagnets.
 5. Anoptical attenuator including a polarizer and an analyzer disposed,respectively, before and after a plurality of Faraday elements, in whicha variable magnetic field is applied to Faraday elements, in such mannerthat the variable magnetic field can change the angle of Faradayrotation of a light beam and control the quantity of light transmitted,said optical axis being in the <111> direction of single crystal ofgarnet, characterized in that three single crystals of garnet ofsubstantially the same thickness having a Faraday effect are used toform Faraday elements and the Faraday elements are arranged in such amanner that a variable magnetic field is applied to one of the Faradayelements, in the direction perpendicular to planes over a rangeextending 5 deg. each to the left and right of the line connecting the(111) plane in the center of the stereographic projection chart with the(−1−12) plane on the outermost circumference or a plane equivalentthereto in the chart, whereas a variable magnetic field is applied tothe remaining two elements, in the direction perpendicular to planesover a range extending 5 deg. each to the left and right of the lineconnecting the (111) plane in the center of the stereographic projectionchart with the (−101) plane on the outermost circumference or a planeequivalent thereto in the chart.
 6. An optical attenuator according toclaim 5, in which said variable magnetic field is a composite magneticfield applied from a pair of magnetic fields at least one of which isvariable.
 7. An optical attenuator including a polarizer and an analyzerdisposed, respectively, before and after a plurality of Faradayelements, in which both a magnetic field parallel to and a magneticfield perpendicular to the optical axis are applied to the Faradayelements, one of the magnetic fields being fixed and the other beingvariable, so that the composite magnetic field thereof can change theangle of Faraday rotation of a light beam and control the quantity oflight transmitted, said optical axis being in the <111> direction ofsingle crystal of garnet, characterized in that three single crystals ofgarnet of substantially the same thickness having a Faraday effect areused to form Faraday elements and the Faraday elements are arranged insuch a manner that a variable magnetic field is applied to one of theFaraday elements, in the direction perpendicular to planes over a rangeextending 5 deg. each to the left and right of the line connecting the(111) plane in the center of the stereographic projection chart with the(−1−12) plane on the outermost circumference or a plane equivalentthereto in the chart, whereas a variable magnetic field is applied tothe remaining two elements, in the direction perpendicular to planesover a range extending 5 deg. each to the left and right of the lineconnecting the (111) plane in the center of the stereographic projectionchart with the (−101) plane on the outermost circumference or a planeequivalent thereto in the chart.
 8. An optical attenuator according toclaim 7 in which the magnetic field parallel to the optical axis is afixed magnetic field generated by permanent magnets and the magneticfield perpendicular to the optical axis is a variable magnetic fieldgenerated by electromagnets.
 9. An optical attenuator which controls theangle of Faraday rotation of a light beam that passes through a singlecrystal of garnet having the Faraday effect by applying two externalmagnetic fields, fixed and variable, from two different directions,characterized in that at least one garnet crystal having a Faradayeffect is used as a Faraday element, and a member for holding theFaraday element in place has a stopper to position the front ends ofyokes of electromagnets that apply the variable magnetic field to andaround the holder, with respect to the direction of field application.10. An optical attenuator according to claim 9 characterized in that themember for holding the Faraday element in place has a pair ofpositioning grooves to position the front ends of yokes ofelectromagnets that apply the variable magnetic field to and around theholder, with respect to the direction of light beam.
 11. An opticalattenuator according to claim 9 characterized in that the yokes of theelectromagnets that apply the variable magnetic field have a front endplane each perpendicular to the direction of the variable field with across sectional area no less than 1.7 times that of the plane of theFaraday element perpendicular to the direction of the variable field.12. An optical attenuator comprising a member formed with a first grooveextending across the optical axis and also formed with an opening opento the first groove along the optical axis, a Faraday element disposedin the first groove in alignment with the optical axis, said memberhaving a pair of second grooves formed on both sides of, and close to,the Faraday element, said second grooves extending across the firstgroove and in the direction normal to the optical axis, and a pair ofelectromagnets that produce a variable magnetic field, said magnetshaving yokes the ends of which are fitted in the pair of second grooveson both sides of the Faraday element, the bottoms of the second groovesserving as a stopper for positioning the front ends of the yokes.